1. Field of the Invention
The present invention relates to an optical filter used for a color image reader or the like, and more particularly to an optical filter capable of eliminating noise caused by a near infrared radiation during a color image reading to thereby adjust an amount of red rays, green rays and blue rays by a single filter.
2. Description of the Related Arts
A color sensor provided at a color image reader for receiving a reflection light from a color document, which is irradiated with light from a light source, i.e., a halogen lamp, to thereby obtain color data of the document, has light receiving portions corresponding to each color of red, green and blue. These light receiving portions respectively have a spectral sensitivity shown in FIG. 1. The color sensor senses each ray of three colors, i.e., red (R), green (G) and blue (B) and reads a ratio of amount of each ray, red, green and blue reflected from the document, obtaining color data of the document.
Actually, the spectral sensitivity curve of the color sensor shown in FIG. 1 shows that the blue rays B and green rays G have sensitivity at the level toward the long wavelength from the wavelength of 650 nm. Therefore, the color sensor senses both the red rays R and the near infrared radiation at the level toward the long wavelength from the wavelength of 650 nm as a blue ray B and green ray G, with the result that the accurate color data cannot be obtained.
Further, each element corresponding to each ray of red R, green G and blue B has different sensitivity as shown in FIG. 1. In addition, the halogen lamp as a light source has no flat spectral intensity with respect to red, green and blue. Specifically, it has a spectral intensity satisfying the following equation of B&lt;G&lt;R. Consequently, the color sensor cannot determine that the reflection light from a blank document having a ratio of R:G:B of 1:1:1 satisfies the equation of R:G:B=1:1:1, making it difficult to obtain the accurate color data.